Mass spectrometry is a common analytical technique used in the physical and biological sciences. Time-of-flight mass spectrometry (TOF-MS) is one mass spectrometry technique used for analytical measurements. TOF-MS has such desirable characteristics as an almost limitless mass range, an ability to provide a complete mass spectrum from each ionization event, and relatively simple operational principles.
A TOF mass spectrometer is composed of an ion injector, a mass analyzer and an ion detector. A packet of ions derived from a sample is input to the ion injector. The packet of ions is typically composed of ions of multiple, different ion species having respective mass-to-charge ratios. An electrical pulse applied to the ion injector imparts approximately the same initial kinetic energy to all the ions in the packet of ions in such a manner that the ions all move in approximately the same direction of travel. The ions of each ion species travel at a respective velocity that depends on the mass-to-charge ratio of the ion species. The ions pass into the mass analyzer, which, in its simplest implementation, is an elongate evacuated chamber. In the mass analyzer, the differing velocities of the different ion species cause the ions of the respective ion species to separate in the direction of travel. At the distal end of the mass analyzer, the ions are incident on the ion detector, which measures the abundance of ions incident thereon within successive narrow time-of-flight windows to produce a time-of-flight spectrum. The time-of-flight spectrum represents the relationship between ion abundance and time of flight. Since the time of flight of the ions of a given ion species is proportional to the square root of the mass-to-charge ratio of the ion species, the time-of-flight spectrum can be converted directly to a mass spectrum that represents the relationship between ion abundance and mass-to-charge ratio. In this disclosure, for brevity, term mass-to-charge ratio will be abbreviated as mass.
The mass resolution in a mass spectrometer is defined as T/2ΔT, where T is the measured time of flight at a given mass, and ΔT is the measured or calculated time-of-flight spread for that given mass. For a TOF mass spectrometer, the square root dependence of the time of flight on the mass dictates that, for large masses, the peak separation decreases inversely with the square root of the ion mass. In recent years there has been a significant increase in applications of mass spectrometry to large biological molecules. Such applications have mass resolution demands that exceed the capabilities of conventional TOF-MS systems. To make TOF mass spectrometers, with their many other desirable characteristics, viable for use in such applications, their mass resolution must be increased.
The mass resolution of a TOF mass spectrometer is proportional to the length of the flight path between the ion injector and the ion detector. A typical TOF mass spectrometer has a linear flight path. Increasing the physical length of such linear flight path until the required resolution is reached would increase the physical dimensions of the instrument beyond those considered reasonable.
A cylindrically symmetric mirror structure such as disclosed in the above-referenced applications to Flory, et al. provides comparatively large flight paths for ions in a mass analyzer, while beneficially reducing the physical dimensions of the mass analyzer compared to mass analyzers with a linear flight path. In cylindrically symmetric mirror structures, the ions from an ion source follow eccentric orbits that slowly precess about an axis of axial symmetry and ultimately are intercepted by the ion detector. The motion in the axial dimension is roughly periodic about the symmetry plane of the cylindrically symmetric mirror structure, located approximately midway between the parallel planar surfaces
While cylindrically symmetric mirror structures beneficially reduce the required physical space without sacrificing resolution compared to mass analyzers with a linear flight path, incorporation of ion sources into such mass analyzer has been difficult.
Accordingly, what is needed is an ion source for a cylindrically symmetric mass analyzer.